Help with late 70's merc 50horse thunderbolt...

Buddy and I took his boat out today after winter storage. Our day was wrecked because the motor would not run properly past 1/2 throttle. The motor runs fine at 1/2 throttle and below. When you try to go past that it sounds like the motor keeps shutting off cylinders. It would chug, then some cylinders would start to fire than they would stop firing again. No back firing.

When we pulled the pluggs out they were ALL soaking wet with gas. So I thought drying them would fix it. Nope, same thing. And if we pulled the pluggs they would be wet again. Tried 3 sets of plugs, all the same result.

Its a late 70's 50 horse merc thunderbolt.

Additional info. Last summer he bought the bought and from day one when you run the motor it ran fine, but when you stopped and fished for a bit when you try to turn the motor over it wouldn't start. You had to prime the bulb everytime you re started the motor. IF you didnt prime it until it was hard, it would not start. The motor was losing its prime after about 10 mins of sitting. We ran it like this all summer because we didnt want to be without a boat while repairing it and ordering parts.

This past winter my buddy took the fuel filter, fuel lines, and fuel pump off to see if he could find a leak. He didn't find a leak so he put it back together again. He didn't try it until today. And the motor held its prime all day. We didnt have to prime it once. So I am guessing the fuel filter, pump or fuel lines were loose and when my buddy put them back on he tightened some stuff up. In order to do this work he had to disconnect the choke and take off some wires. He said he put everything back on correctly using the repair manual. He also replaced the pluggs over the winter.

When he put the fuel pump back together he said he might have damaged the diaphram inside. He siad he didn't think he did, but when he was putting it together he might have.

And lastly, he had half a tank of reg gas left over from the fall. All he did was top the can up with fresh fuel today. No fuel stabilizer.

What I can think off
-old gas. We flushed the lines and tried fresh from the pump gas today. Did nothing. But maybe the old gas over the fuel gummed up his motor?
-Because my buddy might have fixed a vacum leak in the fuel system maybe he needs to re set the high speed jet? I can see the jet might be off because it was set to run with the motor having a vacum leak ...
-he DID damage the diaphragm.
-Something is amiss in the ingition. Coil? Timming? Points? Keep in mind it runs like silk at low speeds and idle and we tried 3 sets of pluggs.



Any help would be appriciated.

I should add because the engines flooding itself when you try to open it up could it also be a choke issues?

First off the high speed jet is NOT adjustable !!---Only the low speed jet is adjustable.-------------I would install a fuel pump repair kit as it sounds like gas is leaking right into the motor.--Could be leaking float valves too.------Also on a 2 stroke motor the carburetors must be verified to be clean, fuel restriction also mean less oil is going into the motor.------Do a spark check to make sure you have spark that will jump a gap of 7/16" on all 4 cylinders.

racerone said:

First off the high speed jet is NOT adjustable !!---Only the low speed jet is adjustable.-------------I would install a fuel pump repair kit as it sounds like gas is leaking right into the motor.--Could be leaking float valves too.------Also on a 2 stroke motor the carburetors must be verified to be clean, fuel restriction also mean less oil is going into the motor.------Do a spark check to make sure you have spark that will jump a gap of 7/16" on all 4 cylinders.

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Thanks!

There is lots of spark, I check this when I dried the pluggs off.

You can't adjust the gap on these plugs. Tough to explain the plugg type, but there is no bendable electrode coming from the threaded part of the plug.

The float valves are in the carbs right?

I guess we'll start with a fuel pump replair kit and clean the cabrs.

The wet spark pluggs were deffinity oil and gas mixed. So I assumed it was getting proper amounts of oil and the motor is not oil injected. Its mixed gas, so how could it be getting less oil?


thanks again

Sorry you mis-understand , you need to establish a 7/16" gap as a test for the ignition system.Most shops have a test set-up so they can test with up to 1/2" gap setting, must be good reason for this .-I know what your sparkplugs look like !----------------Your motor has 2 carburetors and it could be running properly on one.--The other caburetor could be partially plugged and dragging those 2 cylinders along with less fuel and therefor less oil.----When you understand how things work then the trouble shooting becomes " easy "

racerone said:

Sorry you mis-understand , you need to establish a 7/16" gap as a test for the ignition system.Most shops have a test set-up so they can test with up to 1/2" gap setting, must be good reason for this .-I know what your sparkplugs look like !----------------Your motor has 2 carburetors and it could be running properly on one.--The other caburetor could be partially plugged and dragging those 2 cylinders along with less fuel and therefor less oil.----When you understand how things work then the trouble shooting becomes " easy "

Click to expand...

Thanks again. The additional info helped. I will let him know to start for fuel pump repair kit and clean out the carbs..

Well finally got the fuel pump repair kit and installed it last week. Nothing changed. Motor still runs ok until about half throttle and then it floods itself out. The pistons keep flooding to the point where the plugs can't fire.

If I take the pluggs out and dry them and put them back in they stay dry and the engine runs ok. If you take the pluggs out they are still dry. BUT if you try to go past half throttle the motor cuts down to one or two cylinders. Then when you take the pluggs out, most are completly soaked in fuel.

So the motor is still getting too much fuel or no spark past half throttle. Please offer more suggestions.

I should add, we'll be taking the carbs off this week and cleaning them out.

personally (if you re-read my original post) I think the problem is linked to something my buddy did to try to fix the vacum leak. But he followed the manual to a T and swears he screwed notihng up except the fuel pump. Which is no replaced.

What are the results of a compression test ?----You say there is " lots of spark " -------that does not mean anything to me , the question was " will spark jump a gap of 7/16" or more " and that was not answered.------If I was standing beside your motor doing some simple checks I would have the answer in about 10 / 15 minutes.----If you do not answer the question then we are limited in how we can steer you in the right direction. Simple as that.

It sounds to me like your motor is simply over proped for the load. Did this combination EVER run properly (with the present load in the boat)?

Jeff